Warne for me, primarily because:
* He took more wickets in each Test (even though both men averaged around 5 per Test)
* Bowled longer spells in each Test (which is expected of spinners)
* Was the better 'big match' performer, in my mind (1999 World Cup, 2005 Ashes, 2004 Sri Lanka etc)
I also think people under-value Warne because of his bowling average. It gets worse when people rate Kumble. Here's an interesting thread on Kumble in which I posted a while ago.
http://www.cricketweb.net/forum/cricket-chat/34734-murali-vs-kumble-right-now.html#post1428921
Basically I was trying to get people to realise that the best bowlers are the ones who win you the Test match, they're not always the ones that get wickets the fastest, which is generally the criteria people use when judging bowlers (strike-rate, average etc). And Kumble is a better example to use than Warne. Lets say there's a bowler who takes 3-50 from 27 overs. That'll be great for the stats because his average is below 20 and his strike-rate is 54.
Then lets say you have Kumble who takes 6-180 from 65 overs. His average will be around 30 and his strike rate above 60, but Kumble is the reason you win the Test match. He's the reason you win the Test match because while the other bowler took quick qickets, he couldn't keep going after 27 overs, which a lot of pace bowlers can't do. In a nutshell, he can't win you the Test match. Kumble can! Test match cricket isn't always a race for time. You have 5 long days. You need 20 wickets more than you need quick wickets that don't amount to 20.
I use Kumble as an example because I feel Warne was Australia's go-to guy more often than McGrath, although not by much. Warne bowled the longer spells and took slightly more wickets per match IIRC. I really don't think 3 runs difference in average compares to the importance of getting that extra wicket.
I've been a tiny critic of Glen McGrath in the past, which isn't to say he's not one of the greatest bowlers ever. Here were my contentions -
http://www.cricketweb.net/forum/cricket-chat/23845-mcgrath-little-overrated-these-days.html
My main criticisms of McGrath are:
* There have been examples (rare though they are), where I think a bowler with more in his aresenal (pace, swing) would have succeeded in moments when McGrath couldn't take crucial wickets (which wasn't often).
* I really don't think people realise that McGrath wasn't one of the world's best bowlers until around 1998. He was always economical and by 1995 he was effective. But nobody, at the time, rated him in the same league as Ambrose, Akram, Donald etc. It's what I'd call a 'revisionist thing'. Back in those days McGrath was rated highly, but it wasn't until later that he was one of the best. His stats really belied his contributions during those early years. This isn't to say he didn't win Australia matches from 1995-1997. He did and was MOTM in a few games, but generally he wasn't seen as good as some of his other fast bowling peers.
In fact his bowling from 1995-1997 reminded me of his bowling just before the 2006/07 Ashes, when people were saying McGrath was too old and not taking wickets. It's true McGrath wasn't taking many wickets during that period, but really his average didn't suffer much because even then he was very economical. Economical bowlers aren't very useful if they don't take a lot of wickets, but their stats don't suffer. McGrath wasn't quite the threat he would later become.
Personally I rate McGrath's best period as from Perth 2004, when he ripped apart Pakistan, to the time he injured himself at Edgbaston. After that I really don't think he ever returned to his best, despite some significant contributions in his final Ashes series (I'm thinking of the first Test). Gosh he was one of those players that scared the life out of you.
This isn't to say there aren't arguments that McGrath is better than Warne. There are. But generally I think Warne was more important to Australia winning Tests. Obviously it would be stupid to say there was much of a margin between the importance of both men to Australia winning.